The present invention relates to a pneumatic booster which is used in a brake system for a vehicle.
In many vehicles, use is made of a brake system comprising: a pneumatic booster for boosting a depressing force applied to a brake pedal; a master cylinder for generating a fluid pressure in accordance with an output of the pneumatic booster; and a disk brake which receives the fluid pressure from the master cylinder by means of a piston within a caliper and generates a predetermined braking force. Conventionally, for larger vehicles, a tandem type pneumatic booster having two power pistons arranged in tandem is employed in the brake system, because a tandem type pneumatic booster is capable of generating a high output. In recent years, however, study has been made of using a single-type pneumatic booster having a single power piston, with a view to reducing the size of a brake system.
In order to enable the use of a single-type pneumatic booster, it is necessary to increase a ratio of a pressure-receiving area of a cylinder within the caliper in the disk brake to a pressure-receiving area of the master cylinder. That is, efficiency of the master cylinder must be increased by reducing the diameter of the master cylinder to a level which is as low as possible. However, when a single-type pneumatic booster is applied simply by reducing the diameter of the master cylinder, the amount of fluid for operating the disk brake becomes insufficient. This insufficiency of the amount of fluid can be compensated for by increasing the stroke in each of the master cylinder and the pneumatic booster. However, when the stroke in the pneumatic booster is increased, the stroke of the brake pedal is also increased. This lowers brake operability. Therefore, in order to ensure the same brake operability as obtained in conventional brake systems, it is necessary to reduce a pedal ratio (a ratio of a distance between a pivot for the brake pedal and a point at which an input force is applied to a distance between the pivot and a point at which an output is obtained).
When the pedal ratio becomes small, the depressing force applied to the brake pedal becomes large. As a measure for preventing such an increase in the depressing force, it is known to increase a fluid force for advancing a plunger in the booster, which force is generated due to a difference between negative pressure and positive pressure acting on the booster. However, no effective mechanisms have been developed to increase the above-mentioned fluid force.